1. Field of the Invention
This invention relates to a turbocharged engine provided with a supercharger comprising a turbine disposed in the exhaust passage and an impeller disposed in the intake passage.
2. Description of the Prior Art
In turbocharged engines, there has been a problem of lag in the increase in the turbine rpm (turbo lag) during acceleration due to the inertia of the rotating parts. Especially, at low speeds and light loads, the rpm of the rotating part is very low because of low energy of exhaust gas, and when abrupt acceleration of the vehicle is attempted from this condition, the lag in increase of the turbine rpm is considerable and accordingly the lag in increase of the supercharging pressure or the engine output is large, limiting the acceleration performance. If a small turbocharger is used in which the mass of the rotating parts is small in order to reduce the inertia of the rotating parts, the supercharging capacity in the high-speed ranges becomes insufficient, significantly reducing the engine output.
In U.S. Pat. No. 4,207,743, there is disclosed a turbocharged engine which is provided with a bypass passage bypassing the impeller of the turbocharger and in which the intake side or the discharge side of the impeller is closed and the bypass passage is opened when the engine load is light. Thus, in the turbocharged engine, intake air is introduced into the combustion chamber through the bypass passage under intake vacuum without reducing the turbine rpm so that the impeller rpm can be immediately increased when the engine is to be accelerated, thereby reducing the turbo lag.
However, the turbocharged engine is disadvantageous in that since the intake side or the discharge side of the impeller is completely closed during light load operation, the turbo characteristic is apt to exceed the surging line, generating abnormal noises or rotating the turbine in the reverse direction, thereby adversely affecting the durability of the turbocharger.